A variety of mechanisms are available for converting a controlled electrical input signal into a variable, controlled output of an hydraulic device. Typical, such devices are reflected in, for example, the Moog U.S. Pat. Nos. 2,625,136, 2,931,343 and 3,228,423, Paine U.S. Pat. No. 2,832,318, Nelson U.S. Pat. No. 3,001,549, Hawk U.S. Pat. No. 3,430,656, Markson U.S. Pat. No. 2,904,075, Arnett U.S. Pat. No. 3,182,561, Andrews U.S. Pat. No. 3,457,956, Orloff U.S. Pat. No. 3,027,917, O'Brien U.S. Pat. No. 3,323,090, and Lantz U.S. Pat. No. 2,836,154, as well as the McCormick U.S. Pat. Nos. 3,498,329 and 3,516,441. While many of these prior art mechanisms have a certain level of operability, all suffer from significant disadvantages, either operational or economical, or both. Most are quite complex and therefore extremely costly to manufacture, severely limiting their practical utilization. Others have significant operational shortcomings, in terms of non-linearity, inability to stand up under industrial conditions, or the like.